Spirit Grass alone was not enough.
It wasn't that the pasture in the mountains wasn't plentiful, but rather because the Spiritual Energy contained within it was too dense.
It wasn't just the pasture that contained Spiritual Energy; in addition, there was the energy from the heavens that gathered here.
The chickens, ducks, and geese were merely ordinary beings of flesh and blood who, despite their eagerness to eat, could often only take a few bites before their stomachs swelled and they became so bloated that they struggled to walk. Thus, separate grain fodder had to be prepared for them.
As for those precious birds, there was no need for him to prepare food.
They could fly farther, wander the cosmos, run faster, and sprint through rivers and seas. Naturally, their feeding grounds were far more extensive.
Furthermore,
they wouldn't even touch the ordinary fodder; their food came from all corners of the earth, but it most certainly did not include this common feed for poultry.